Update the ci_*.sh scripts and move them to the more conventional
ci/ directory. Update .appveyor.yml and .travis.yml, as well as
the AUTHORS file, accordingly.
Speed up ci_cmake.sh: add "-DPNG_TESTS=OFF" to the list of CMake
variables if CI_NO_TEST is true.
Remove "sudo: false" from .travis.yml.
Refactor .appveyor.yml.
Exclude the branch "libpng16" from testing. It's identical to "master".
Also exclude the other (non-current) "libpng[0-1][0-7]" branches.
Parallelize the builds and the unit tests.
For Travis CI, update the build matrix as follows:
* On Linux, use both clang and gcc.
* On macOS, use clang only, with and without Xcode.
For AppVeyor CI, add a build matrix as follows:
* On Windows, use Microsoft Visual Studio 2019 on x86, x64 and arm64.
* Also on Windows, use MSYS2 on x86 and x64.
Use the new scripts/ci_*.sh; remove the old scripts/travis.sh.
